1. Field of the Invention
This invention relates to a projection exposure apparatus for transferring fine patterns onto a wafer and, more particularly, to a projection exposure apparatus which transfers fine patterns by using a beam in the range from the X-ray region to the vacuum ultraviolet region.
2. Description of the Related Art
Recently, with the development of semiconductor devices having higher packaging densities, there is a demand for projection exposure apparatuses having higher resolutions to transfer finer circuit patterns. Generally, as is well known, the resolution of a projection exposure apparatus is higher if the numerical aperture of the projection optical system is larger or if the wavelength of illumination light is shorter. However, if the numerical aperture is increased, large-incident-angle components in illumination light incident upon a wafer are increased and, accordingly, the depth of focus is reduced, resulting a deterioration in pattern transfer accuracy.
For this reason, many trials have been made to improve the resolution of projection exposure apparatuses by using as illumination light a beam of short-wavelength rays such as X rays. However, if the wavelength of a beam is reduced, the beam can be absorbed in a material more easily. For example, it is difficult to use an image optical system using transmission lenses with respect to a beam in the range from the X-ray region to the vacuum ultraviolet region as in the case of conventional exposure apparatuses using a mercury lamp or the like as a light source. It is necessary to use a reflection type imaging optical system in such a case. For example, Japanese Patent Laid-Open Nos. 63-18626 and 63-312638 disclose reflection type imaging optical systems for transferring mask patterns onto a water by using X-rays.
Conventionally, however, synchrotron radiation light used as an illumination beam is treated substantially as a point light source. Therefore, the spatial coherence is increased. As a result, if fine complicated patterns formed on a mask are transferred, a deterioration occurs in a projected image due to the influence of interference between adjacent patterns.